The plasmodium of the slime molds (Class Myxomycetes) can be considered to be a giant multinucleate amoeba (syncytium). As such it is excellent material to use to study genetics of cell fusion. The purpose of this work is to gain an understanding of the genetic and biochemical factors which control fusion or nonfusion between two cells. In general, cell fusion is a rare process in nature except during sexual reproduction. In slime molds asexual cell fusion (plasmodial fusion) is also relatively rare even among siblings. However, pieces of a fragmented plasmodium will readily coalesce on contact to reform one individual. Plasmodial fusion is thus a problem on the recognition of self and nonself. By using the standard Mendelian techniques for genetic analysis we have determined that at least 11 loci (fusion loci), each with dominant and recessive alleles, control fusion of plasmodia. Two plasmodia must be phenotypically identical at all 11 loci in order to fuse with each other. In this project we will extend the fusion studies to include more strains of the slime mold Didymium iridis. The purpose of these studies is to provide a firm genetic base from which we can initiate biochemical studies on the mechanism of cell fusion. Preliminary biochemical studies will be carried out in an attempt to identify the gene products of the fusion alleles which may appear at the plasma membrane. Immunological methods will be used to determine if any differences can be detected in plasma membranes of plasmodia with different phenotypes at several fusion loci.